[modules] When determining whether a name from a module replaces a name we

[modules] When determining whether a name from a module replaces a name we
already have, check whether the name from the module is actually newer than the
existing declaration. If it isn't, we might (say) replace a visible declaration
with an injected friend, and thus make it invisible (or lose a default argument
or an array bound).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@228661 91177308-0d34-0410-b5e6-96231b3b80d8